1. Field of the invention
The invention relates to the field of thermal waste treatment. It relates to a process for reprocessing slag and/or ash from the thermal treatment of refuse.
2. Discussion of the background
For the thermal treatment of waste/refuse, in addition to the classic waste incineration, many processes are also known in which the refuse is degassed and/or gasified, or combinations of these known processes are employed. The solid reaction products formed in these processes can be further treated in various ways, e.g. thermally, the products which are no longer utilizable then being landfilled. For environmental and cost reasons, attempts are being made to keep these unavoidable amounts of residues as small as possible and to reprocess the slags or ashes so that they can be used again as valuable materials or inert materials.
WO 93/17280, for example, discloses a process for fusing combustion residues in slag, in which the waste is first subjected to low-temperature carbonization in a low-temperature unit and then, with conjoint use of low-temperature carbonization materials and gases, complete combustion with slag liquefaction is carried out in a high-temperature unit at approximately 1200-1400.degree. C. The end product is a completely burnt liquefied slag which can be allowed to solidify in any desired form. Firstly, this slag has a low loss on ignition, i.e. a low content of unburned constituents, secondly, highly toxic hydrocarbon compounds, such as dioxins and furans, are below the detection limits, and heavy metal compounds are said to be incorporated in the slag in virtually insoluble form. These advantages are opposed by the following disadvantages:
1. For domestic refuse incineration, the grate firing process is currently usually used. In this process the refuse is moved mechanically over a horizontal or inclined plane and simultaneously combustion air, which enters the refuse bed from below through the grate, flows through it. The incombustible portion of the waste is discharged from the combustion plant as grate ash or slag. Low-temperature carbonization of the refuse and subsequent slag liquefaction, as in the abovementioned process, is not possible in these widespread existing plants. PA0 2. To fuse the slag in the rotary kiln, very high temperatures are necessary, so that high-grade and expensive brick lining material must be used. PA0 3. In the fusion process, the environmentally harmful heavy metals nevertheless pass in an uncontrolled manner into the environment and cannot be recovered. PA0 4. The high energy consumption, required for the liquefaction, is a further disadvantage.
EP 0 372 039 B1 discloses a process for reprocessing slag from waste incineration plants, in which the slag is discharged dry from the incineration furnace, is subjected to a coarse cleaning (removal of unburned coarse material and magnetic components), and then the coarsely cleaned slag is separated into at least two fractions and all particles which are smaller than 2 mm are assigned to one of these. This process is based on the finding that the fine fraction comprises the majority of the pollutants originally present in the slag. The fine fraction is fed to a special treatment, while the coarse fraction is suitable as building material, for example.
A further development of this process is disclosed in EP 0 722 777 A1. There, a process is claimed for reprocessing slag from refuse incineration plants, in which the crude slag, after passing through the firing grate, is separated directly and without prior quenching in a waterbath into at least two fractions, and these two fractions are further treated separately, the coarse fraction being fed to a wet slag remover. Features of the process are that the first fraction having a particle size less than 80 mm, preferably less than about 32 mm, is separated off in a first screening stage, that the screening oversize is fed to the wet slag removal, that the screen undersize and, if appropriate, the material falling through the firing grate, is fed to a second screening stage for separating off the fines 0 . . . 2 mm, that the screen oversize of the second stage, if appropriate after removal of metallic materials and inert materials, is mechanically comminuted, and the screen undersize of the second stage is fed to a special treatment, e.g. a melting furnace. In this melting process, carried out for example in an arc furnace, a vitreous readily landfilled product and a metal concentrate are produced (see F.-G. Simon and K.-H. Andersson: InRec-Verfahren--Verwertung von Reststoffen aus der thermischen Abfallbehandlung [InRec process--utilization of residues from thermal waste treatment], ABB Technik 9/1995, pp. 15-20). This reprocessing process has been used previously in practice for the slag from grate incineration furnaces and has proved to be useful there. Disadvantages are on the one hand the high costs which result from the use of the arc furnace, and on the other hand the many classification and comminution stages for the slag or ash.
Various volatilization processes are known from the metallurgical industry for metal production, for example the rolling process. This process which operates with reducing conditions has the purpose of producing heavy metals such as lead, zinc and cadmium, in the form of highly enriched fly dusts (see Ullmann: Enzyklopadie der technischen Chemie [Encyclopedia of Industrial Chemistry], 4th edition, p. 429). As starting material, use is made in the rolling process of lean, oxidic, non-readily-beneficiated zinc-lead ores, zinc-lead-containing fly dusts and foundry waste products. These metalliferous starting materials are mixed with a reducing agent, e.g. coke or anthracite, and fed to a rotary kiln. They travel through the furnace, being heated in the course of this, until the volatilization begins at about 1000.degree. C. The volatilization reactions (reaction of the solid metal oxides with the admixed carbon to form gaseous metals and carbon monoxide) proceed in the solids layer of the rotary kiln, which solids layer is constantly recirculated. In the gas space which is above it and has an oxidizing atmosphere, the volatilization products are then reoxidized. These oxidation products from the gaseous phase are very finely particulate, so that they are entrained by the flue gas, transported out of the furnace and are finally separated off after the flue gases have cooled. The slag low in metals is discharged from the furnace, cooled and then placed on a slag heap.